The unusual combination of anisotropic and fluid behaviour of liquid crystals has resulted in their use in a large number of electro-optical switching and display devices. In these, their electrical, magnetic, elastic and/or thermal properties can be used for changes in orientation. Optical effects can then be achieved, for example, with the aid of birefringence, the inclusion of dichroically absorbing dye molecules ("guest-host mode") or light scattering.
In order to satisfy the constantly increasing practical requirements in the various fields of application, there is a constant demand for novel, improved liquid-crystal mixtures and thus also for a large number of mesogenic compounds of a very wide variety of structures. This applies both to the areas in which nematic LC phases (for example TN="twisted nematic", STN="supertwisted nematic", SBE="supertwisted birefringence effect", ECB="electrically controlled birefringence") are used, and in those using smectic LC phases (for example ferroelectric and electroclinic).
Many of the compounds which are suitable for LC mixtures can be described by a structure principle (building plan) [see, for example J. Am. Chem. Soc. 108, 4736 (1986), structure I; Science 231, 350 (1986), FIG. 1A; J. Am. Chem. Soc. 108, 5210 (1986), FIG. 3] in which nuclei of cyclic compounds--aromatics, heteroaromatics, but also saturated ring systems--are linked to alkyl side chains which are straight-chain or substituted in the chain by small groups (for example methyl or chlorine) and are thus branched.
Compounds which contain a terminal cyclopropyl-substituted alkyl chain as a partial structural element are described, for example, in U.S. application Nos. 3,948,961, 3,966,969 and 4,014,922 (Henrick et al.). The compounds are said to be suitable as insecticides, but a liquid-crystalline behaviour is not reported. The compounds listed in these publications differ from those defined below, in particular through the fact that they are always linked to the molecular radical via an ester function and in this molecular radical, although having aromatic ring systems, do not contain any heteroaromatic ring systems.
EP-A 0,244,129 relates to 2,2-dimethylcyclopropane derivatives which are optically active and are linked to one of the known mesogenic radicals via a --CO--O--CH.sub.2 --, --O--CO-- or --CH.sub.2 -- bridge (the cyclopropyl ring is located at the right-hand end of the particular bridging member). These compounds are said to be suitable as components for ferroelectric LC mixtures (spontaneous polarization of up to 11 nC/cm.sup.2).